The present invention relates to content-protection systems, and more specifically, to traitor tracing of anonymous attacks.
The transition of many types of media from analog to digital offers new advantages to the consumer in quality and flexibility. Also, there is an increasing use of global distribution systems such as the Internet for the distribution of digital assets, including music, film, computer programs, photographs, games and other content. These trends have made it easy to produce and distribute flawless copies of content by content providers. Unfortunately, there is also a concurrent increase in the unauthorized copying, or pirating, of digital content, which has caused considerable economic losses to content providers. Effective countermeasures are important to the viability of businesses engaged in the distribution of digital media.
Piracy is a major concern and expense for content providers. To this end, industry consortia such as the 4C Entity (<www.4 centity.com>) and AACSLA (<www.aacsla.com>) have been formed. These groups are licensing agencies that provide content protection tools based on Content Protection for Recordable Media (CPRM) and Advanced Access Content System (AACS), respectively. CPRM is a technology developed and licensed by the 4C group, comprising IBM, Intel, Matsushita, and Toshiba, to allow consumers to make authorized copies of commercial entertainment content where the copyright holder for such content has decided to protect it from unauthorized copying. AACS is a follow-on technology for the same purpose, under development by a group comprising IBM, Intel, Matsushita, Toshiba, Sony, Microsoft, Warner Brothers, and Disney.
In the AACS content protection system, devices such as DVD players are assigned a set of keys and a common key is used to encrypt the content. A pirate attack in this system may occur when the attackers redistribute the common content encrypting key or the plain content to avoid being identified. This type of an attack is called an anonymous attack. In an anonymous attack, an attacker, or group of attackers, tries to hide their secret device keys and operate anonymously. In this attack, the attackers instrument their devices and collude to build a pirate copy of the decrypted plaintext content or the decryption key itself. The attackers can then redistribute either the plaintext content, or the decryption key.
The devices (or the owners of the devices) who are involved in the piracy and redistribution are called “traitors.” Traitor tracing is the forensic technology used to identify the traitors who have been involved in the piracy attack. To do traitor tracing for anonymous attacks, content may be divided into multiple segments and some of the segments may be chosen to have multiple variations. A digital watermark is one way to build these variations. More importantly, those variations are not only differently watermarked, but also differently encrypted. During playback, each device can only decrypt exactly one variation at each segment. The differently watermarked and encrypted variations effectively build different content versions. Each different playback path becomes one version. The recovered pirated variation encrypting keys, or the movie version, can be linked back to the actual devices (i.e., traitors) who were assigned those versions.
There are some practical issues with the above-described traitor-tracing system. First of all, because the variations take extra space on the disc (bandwidth) during communication, the number of variations cannot be large. However, in practice, the number of devices a system needs to accommodate may be very large, e.g., in the billions. These are conflicting requirements. To address this issue some prior systems utilize two level of assignment, namely “inner code” and “outer code”. The inner code assigns the variation for each segment inside the content, which may be a movie. This assignment effectively creates multiple movie versions, each version becoming a symbol for the outer code assignment. The outer code assigns the movie versions (symbols) among a sequence of movies. This assignment solves the extra-bandwidth requirement by having a small number of variations at each segment, while still managing to support a large number of devices.
A second practical issue relates to the actual traitor detection. The problem is that attackers collude in the attack and may mislead the tracing agency to erroneously incriminate innocent devices. The collusion attack creates an inherent difficulty in terms of tracing. After the above-described practical assignment is done, a straightforward approach to detect colluders might be to score every device and incriminate the highest scoring devices. In some prior systems, more efficient tracing algorithms are employed which use a set-cover algorithm to detect coalitions of pirates all together instead of one by one. In these systems, when the number of traitors becomes large, the traceability decreases. Hence, prior-art systems using a set-cover tracing algorithm may work fine when the number of traitors is smaller than q, with q being the number of symbol variations. When the number of traitor exceeds q, the traceability degrades significantly. When the number of traitor reaches q log q, where the coalition gets to know every symbol, the scheme may be nearly broken.